I cloned a project from heroku which was developed by another developer of my company. I cloned that project from heroku using:
heroku git:clone -a "project name" command. I added some new features to that project and fixed some bugs as well. Everything is working quite well in my local server now. But, now, when I try to push the changes to heroku again using:
git push heroku master it is showing the following error :

Permission denied (publickey).
fatal: The remote end hung up unexpectedly

Well, when I try to add it remotely using : git remote add heroku git@heroku.com:{project_name}.git then it shows: fatal: remote heroku already exists.

What is the exact problem? Could anyone please help? Thanks in advance for any kind of help and clue.